Process for producing steel rods for prestressing concrete



Sept. 25, 1956 D. H. LEE 2,764,514

PROCESS FOR PRODUCING STEEL RODS *FOR PRESTRESSING CONCRETE Filed June28, 1954 v 2 Sheets-Sheet 1 STRESS Sept. 25, 1956 D. H. LEE 2,764,514

PROCESS FOR ODUCING STEEL RODS FOR PRES SSING CONCRETE 2 Sheets-Shet 2Filed June 28, 1954 United States Patent PROCESS FOR PRODUCING STEELRODS FOR PRESTRESSING CONCRETE Donovan Henry Lee, London, EnglandApplication June 28, 1954, Serial No. 439,805

Claims priority, application Great Britain October 17, 194-9 7 Claims.(Cl. 148-123) This invention relates to high tensile high carbon steelbar, rod or wire more particularly for use .in the prestressing ofstructural materials, such as concrete, cast iron and mild steel, andhas for its object the provision of an improvement in the production ofsuch bar, rod or wire by which the steel is given desired physicalproperties. This application is a continuation-in-part of my earlierapplication, Serial No. 405,427, filed January 21, 1954, now abandoned,which in turn is a continuationin-part of my application, Serial No.181,863, filed August 28, 1950, now abandoned.

It will be appreciated that the bar, rod or wire hereof differs only indegree of thickness, if at all, and to avoid use of alternativeexpressions the term bar is used in the claims as generic to suchshapes.

It is known to stretch mild steel (carbon content up to say, 0.3%) forthe purpose of raising the yield point and reducing the ductility asmeasured by the elongation, and it is also known to stretch and twistsimilar mild steels in various ways, for example stretching and twistingsteel bars so that the outer surface receives more cold working than theinterior due to the applied torsion. Such treatment can only be employedsatisfactorily with mild steels which are not suitable for efficient usein the prestressing of concrete, where a very high ultimate strength isessential. For prestressed concrete work, high carbon steels (carboncontent of 0.5% and over) can only be utilized, more particularly highcarbon steel alloys of particularly high, ultimate strength properties.

Such high carbon steels or steel alloys initially have a lower ductilitythan mild steels, and therefore to subject them to any combination ofstretching and twisting (as used in the case of mild steels) is notpractical owing to their liability to structural damage or deteriorationas a result of the cold work produced on the external surface bytwisting added to thatproduced by stretching; this may, for example, bepartly due to the surface of such high carbon steels being air hardened.With the limited amounts of cold work that are therefore possible withthese high carbon steels, simple stretching which imparts a uniformamount of cold work to the steel throughout its whole section leads toless risk of brittle fracture at this stage. Furthermore, forprestressed concrete work it is very desirable that all bars should betested before being used and cold working by controlled simplestretching has the additional important advantage that any physicaldefect in the bar is immediately disclosed although perhaps not visible.

There are, however some high carbon steels in which the yield pointcannot be raised by cold stretching, as immediately the yield point isexceeded in one part of the bar, fracture follows. It is thereforenecessary to use only a steel in which the yield point of the hot rolledbar before stretching is distinct and definite, so that during theactual stretching operation immediately one part of the bar reaches theyield point the cold working in that part immediately raises the yieldstress there, and the cold work thus spreads progressively to all partsof 2,764,514 Patented Sept. 25, 1956 ice the bar. To achieve. this, itis necessary initially to apply the stretching force so that the yieldpoint of the as-rolled bar is reached gradually, and thereafter isexceeded to between and of the ultimate strength of the bar. As anexample, a high carbon alloy steel, having a composition of carbonbetween 0.5% and 0.65%, silicon between 1.5% and 2.2% and manganese upto about 1.5% is particularly suitable for such treatment since in theas-rolled condition there is a pronounced yield point around 40-45 tonsper sq. in.

As a further example a high carbon steel having a combined chromium andsilicon content of up to about 2.5% can also be used if great care istaken to observe the treatment requirements described hereafter. Such asteel may have a composition falling within the ranges of carbonfrom0.35 to 0.65%, manganese up to 1.5% and a combined chromium and siliconcontent up to 2.5%, of which the silicon content is up to 0.5%. Steel ofeither of these compositions after rolling to form the rod can have anultimate strength of about 70 tons'per square inch and a yield strengthabout 0.6 of its ultimate strength. By stretching according to theinvention, such a rolled steel bar about 3 to 5% of its original length,the stress-strain curve for the steel after stretching will show anincrease in the length of the straight part of the curve such that thereis practically no curvature between zero stress and a stresscorresponding to.0.7 or more of the ultimate strength of the steel. Thisform of stress-strain curve of the steel after stretching indicatesphysical properties of the steel which are particularly suitable forprestressedconcrete where ductility is not particularly advantageous,but absence of creep of the steel at high stresses is important.

As an example it is desirable that there shall be no creep at constantlength at a working stress which may be of the order of 60% to 70% ofthe ultimate strengflt of the steel and'by the method now described thisresult is obtainable.

It is to be appreciated that the percentage stretch of 3 to 5% as givenabove is a suitable percentage stretch for the type of steel referred toabove, and that with.

scribed. By this means bars subjected to stresses, during transport andhandling, after the processing described, which exceed that of thestraight part of the stress strain curve, receive more cold work insteadof being liable to incipient fracture as can very readily occur withsteel having the physical properties of wire as at present used.Moreover, for prestressed concrete work it is desirable, and is one ofthe purposes of this invention to provide, a steel in which the finalphysical properties are as closely uniform as possible, notwithstandingthe fact that, with all skill shown in the making of the steel it is nota practical possibility to maintain uniformity of the desired selectedchemical composition.

The stretching of the rod provides a form of cold working of the steelwhich increases the yield stress such that the marked yield point, onthe stress-strain curve appropriate to the steel before stretching,disappears and the stress-strain curve after stretching shows apractically straight portion to a much higher stress.

According to the invention, for the prestressing of structural materialssuch as concrete, cast iron or mild steel, high carbon steel bar of thetype indicated is subjected to controlled single linear stretching toraise the proof stress and reduce the ductility. By simple stretching ismeant stretching with the avoidance of any twisting or applied torsionaldistortion.

The invention also consists in preconditioning hot rolled high carbonsteel rod for the prestressing of structural materials such as concrete,cast iron or mild steel, by subjecting the rod to a controlled coldsimple linear stretching until the yield point is determined to havebeen raised to over 70% of the ultimate strength and the ductility asmeasured by elongation on unit length (e. g., an 8 inch gauge length) is'not 'less than 6% nor more than such that creep at constant length isthereby rendered negligible at 60% (about 80,000 p. s. i.) of saidultimate strength.

Further, in accordance with this invention, high carbon steels of thetype described are subjected to a simple linear stretching to a residualelongation which, after the elastic recovery has occurred, issubstantially inverse to the equivalent carbon content. carbon contentis meant the percentage of carbon plus the proportion of the silicon,manganese, chromium or other elements present, expressed as a percentageof carbon in known manner as effective in raising the ultimate strengthin the as-rolled condition. The use of the equivalent carbon content hasbeen found, in practice, to be a convenient method of determining thestretch to 'be given to any particular steel, to obtain the optimumproperties of proof stress and ductility, but the variation of thepercentage stretch inversely to the equivalent carbon content is part ofthis invention whereby steels of slightly varying chemical analysisafter stretching have a practically uniform 0.2% proof stress, andtherefore in a given construction all bars have essentially the samephysical properties and give virtually identical performances.

Further, since stretching of high tensile steels of the high carbon typedescribed cannot ordinarily be carried out except under veryfavourable'conditions of temperature and ageing, further precautions arerequired. For example,'in the cooling of billets prior to the rollinginto bars, hydrogen may become occluded, in which event brittle fracturemay follow immediately on subjecting bars to cold working. Therefore,billets of high carbon steel are preferably subject to treatment toreduce the occurrence of occluded hydrogen, for example, controlledcooling to allow any occluded hydrogen to escape.

However, even with such precautions, steels with 0.5%

carbon or more may still have occluded hydrogen in the bar after rollingfrom the billet and retain a consequent marked susceptibility to brittlefracture during the cold stretching. For example under low temperatureconditions steels of this high carbon type, even those in which thephosphorus content is quite low, are sensitive to cold working.Therefore supplementary to the procedure described above, in coldweather or at other times when found desirable, the bars are subjectedbefore stretching to a heat treatment in water for a period up to 48hours to give them an artificial ageing. Stretching may be effectedwhile the bars are still warm from this treatment.

Since the propagation of any minute crack in the surface of the steel isclosely related to the notch sensitivity, and the latter is increasedthe lower the temperature of the steel when being stretched, it is verydesirable to warm bars in cold weather to not less than about 50 F. Thisis also desirable although it may be known no minute surface crackingexists since minute slag or other inclusions or surface irregularitiesmay have the same effect of being stress raisers.

Further as cold working becomes increasingly ineffective the higher thesteel temperature at the time, it is desirable to limit the maximumtemperature so that it does not exceed the lower inter-crystallinetransition By equivalent reduce ductility out.

temperature. The latter varies with the chemical composition of thesteel and also with any impurities but is generally below 300 F.Assuming no impurities present a stretching temperature range of F. to200 F. is generally satisfactory.

In what follows there is described with the aid of drawings stressstrain curves and particular examples of steel treated and stretchedaccording to the invention, and apparatus for carrying out thestretching.

In the accompanying drawings:

Figure l is a diagram showing stress strain curves produced at variousstages of treatment and stretching of a particular steel according tothe invention,

Figure 2 is a diagrammatic view in perspective of one form of apparatussuitable for stretching and testing steel rod or wire according to theinvention, whilst Figure 3 is a detailed view of part of one of theclamps used in the apparatus of Figure l. I

Referring to Figure 1, curve a shows the typical properties of a steelin its as-rolled condition, which has carbon and other optional elementsin sufiicient proportion to give an ultimate tensile strength of 140,000p. s. i. or over.

The brittleness of this steel immediately after rolling, shown by curvea, is particularly characterised by a small elongation up to the point bof fracture, which will vary with known factors such as chemicalcomposition and with lesser known factors, which this invention is moreparticularly concerned, including rolling strain, and occluded hydrogen.

After the pre-treatment disclosed in this specification and prior tostretching, the stress-strain curve 0 for the steel shows increasedelongation. Without such pretreatment the stretching of steel of thistensile strength is impossible without sudden fracture or except as theresult of fortuitous circumstances not deliberately repeatable.

After the pro-treatment and the stretching according to the invent-ionthe stress-strain relationship is typically represented by curve d.

It will be appreciated that the percentage of stretch can be varied sothat the desired ratio of proof stress e to ultimate strength isobtained and for the prestressing of concrete this is between 0.7 and0.9 in the case of a 0.2% proof stress, over 120,000 p..s. i.

The following examples are given in order to make more clear .theprocedure, according to the invention, necessary to obtain steel barswith the desired physical properties.

In the examples the bars are assumed to be of steel having a carboncontent sufficient to give an ultimate strength of 140,000 p. s. i. orover, or alternatively carbon with other elements which together havethe same result.

In Example lthesteel has no alloying elements which and it is also fullykilled. The billets or bars of such a steel are first subjected tocontrolled cooling to remove rolling strain andoccluded hydrogen. Thestretchings of the bars mayfthen be carried out if the ambienttemperature is F. .or over; if not immersion in heated water for 1 houris normally suificient.

'In Example 2 the bars are of a steel of 140,000 p. s. i. ultimatestrength, an alloy containing silicon and manganese as describedearlier. It isassumed that controlled a cooling of billets or barscannot or has not been carried The bars are first aged by storage in airat a temperature and time depending on the expected, or determined,hydrogen content of the steel. On the average this will be 4 weeks at 60F. after which they can be stretched in accordance with the invention asdescribed earlier.

In Example 3 the steel bars are of any alloy composition :but having acarbon content over 0.50% and giving 140,000 p. s. i. ultimate strengthor over. To enable stretching of these bars in any climatic conditionwithout previous controlled cooling or ageing in air for several weeks,the bars are immersed in hot water for a period of 4 to 20 hours at 200F. down to !100 F., the time and temperature being approximatelyinterchangeable and greater time or temperature being provided whereoccluded hydrogen is believed, or known to be greater than normal.

Such treated bars are then stretched while still warm, that is at atemperature between 60 and 200 F; the lower temperature being sufficientfor safe stretching being above the critical limit of 50 F. mentionedabove.

It will be appreciated from the foregoing that, notwithstanding thepractice in the steel industry to carry out cold working by stretchingand twisting in various ways to give a raised yield point to the steeland sometimes to raise the ultimate strength of high carbon steels forthe purpose of prestressed concrete work, those methods are inadequate.Several years of experiment and research were needed to discover .aprocedure that would enable high strength high carbon steel bars to bemade specifically suitable for prestressed concrete work to eliminatebrittle fractures during processing and also, incidentally, to ensurethe testing of every bar against the possibility of invisible ornon-apparent defects which might lead to incipient fracture when the barwas subsequently tensioned in use.

In order to effect the stretching of the steel bar in accordance withthe invention the bar either in a single length or .a number of lengthstogether, may be clamped at its or their ends and a force applied to oneor both clamps by means, such as for example hydrallic cylinder and rammeans, which may be controlled to give the rod or wire the predeterminedextension at which the steel obtains the desired physical properties.

A convenient apparatus for carrying out the controlled simple linearstretching of high carbon steel rod is shown in Figures 2 and 3, inwhich the apparatus comprises two side or strut members a which arejoined together at one end by an end member b. Located between the sidemembers are two clamps c and d which are capable of movement between andlongitudinally of the side members a. Rollers 2 carried by the clampsride upon the upper surface of the side members in order to support theclamps and facilitate their movement upon the side members.

The clamps c and d are provided with opposed tapered slots f and g, andin each slot there is a pair of wedgelike dogs 11 which have serratedinner surfaces for gripping the ends of the steel rod i to be stretchedbetween the clamps.

Arranged for sliding movement upon and between extensions j of the sidemembers a is a cross head k which is connected to the clamp d by fourdraw bars indicated at m. As shown the force for stretching the rod i issupplied by hydraulic cylinder and ram means of which the cylinder n issecured to the end member b and the ram acts upon the crosshead k.

The position of the clamp 0 between the side members a is madeadjustable in order to provide for variations in length of the steelrods i to be stretched, and for this purpose the clamp c is provided atits ends with hinged sprags or pawls p (see Figure 2) which engage toothor rack formations q on the insides of the side members a.

With the arrangement above described the force [applied by the ram 0 tothe crosshead k is transmitted by means of the draw bars m to the clampd. Simultaneously the reaction of the thrust of the ram upon thecrosshead is taken by the end member b and spread thereby on to the twoside members a which in turn transmit it to the clamp c by means of thesprags 2. Thus the rod i is stretched by a force applied at each end ofthe rod by the clamps c and d.

Suitable pointer and scale mechanism, for example upon the clamps andside members respectively, may be provided for measurement of thestretch of the rod i, and in addition suitable pressure measuring meansmay be provided for measuring the pressure in the hydraulic cylinder orthe supply main leading to it. With the aid of the readings from thesemeasuring means a careful control of the strteching of the steel bar canbe obtained.

The invention is primarily suitable for the production and coincidenttesting of steel bars for use in prestressed concrete, but it may alsobe used for the production and coincident testing of steel bars to beused for other purposes, such as tie bars suitable as wind bracing forthe frameworks of buildings and bridges and also as the tie bars ofsheet piled walls for docks and river walls.

I-claim:

1. A method of producing a high tensile steel memher for prestressingconcrete and other structures which comprises providing a rolled barcontaining 0.35% to 0.65% carbon, manganese up to 1.5% and silicon up to2.2%, said bar having an ultimate strength exceeding 140,000 p. s. i., a0.2% proof stress of over 120,000 p. s. i. and substantially no creep ata stress .of about 60% of the ultimate strength, said bar beingnon-stretchable in the as-rolled condition, the steps comprising agingsaid bar to remove occluded hydrogen and then stretching the bar byslowly applying tension to the bar so as to approach and exceed theyield point, whereby a yield point flow spreads along the entire lengthof the bar when the yield point is reached, the application of tensionbeing continued until the bar has been given a permanent elongation offrom 3% to 5% and said proof stress is obtained.

2. The method as defined in claim 1 wherein the bar is aged to removeoccluded hydrogen by soaking in water at a temperature exceeding F.

3. The method as defined in claim 1 wherein the bar is aged to removeoccluded hydrogen by maintaining it for a period after rolling of atleast one week in air at a temperature exceeding 50 F.

4. The method as defined in claim 1 wherein the aging step comprisesslowly cooling a hot bar from a temperature substantially higher down toabout 300 F. and then stretching said bar.

5. The method as defined in claim 1 wherein the composition of the metalstretched comprises carbon from 0.35% to 0.65%, manganese up to 1.5% anda combined chromium and silicon content up to 2.5% of which the siliconcontent does not exceed 0.5%.

6. The method as defined in claim 1 in which the metal composition hascarbon in the range of 0.5% to 0.65%, silicon 1.5% to 2.2% and manganeseup to 1.5%.

7. The method as defined in claim 1 in which the aged bar is heated tothe range of 50 to 300 F. and then stretched.

References Cited in the file of this patent UNITED STATES PATENTS2,136,538 Borwick Nov. 15, 1938 FOREIGN PATENTS 568,608 Great BritainApr. 12, 1945 616,803 Great Britain J an. 27, 1949 681,185 Great BritainOct. 22, 1952

1. A METHOD OF PRODUCING A HIGH TENSILE STEEL MEMBER FOR PRESTRESSINGCONCRETE AND OTHER STRUCTURES WHICH COMPRISES PROVIDING A ROLLED BARCONTAINING 0.35% TO 0.65% CARBON, MANGANESE UP TO 1.5% AND SILICON UP TO2.2%, SAID BAR HAVING AN ULTIMATE STRENGTH EXCEEDING 140,000 P.S.I., A0.2% PROOF STRESS OF OVER 120,000 P.S.I. AND SUBSTANTIALLY NO CREEP AT ASTRESS OF ABOUT 60% OF THE ULTIMATE STRENGTH, SAID BAR BEINGNON-STRETCHABLE IN THE AS-ROLLED CONDITION, THE STEPS COMPRISING AGINGSAID BAR TO REMOVE OCCULDED HYDROGEN AND THEN STRETCHING THE BAR BYSLOWLY APPLYING TENSION TO THE BAR SO AS TO APPROACH AND EXCEED THEYIELD POINT, WHEREBY A YIELD POINT FLOW SPREADS ALONG THE ENTIRE LENGTHOF THE BAR WHEN THE YIELD POINT IS REACHED, THE APPLICATION OF TENSIONBEING CONTINUED UNTIL THE BAR HAS BEEN GIVEN A PERMANENT ELONGATION OFFROM 3% TO 5% AND SAID PROOF STRESS IS OBTAINED.